Electrostatics 1
History The word electricity comes from the Greek elektron which means amber. The amber effect is what we call static electricity. 2
ELECTROSTATICS the study of electric charges, forces and fields Static Electricity is Stationary Electricity or Accumulation of charge
History Two types of charges exists, arbitrarily named POSITIVE and NEGATIVE By Benjamin Franklin So what is positive and what is negative?
We know that charged particles exist in atoms Electrons are responsible for negative charge and Protons for positive charge Benjamin Franklin did not know about the existence of these particles but, he did investigate the behavior of static discharge and lightning.
Ben knew that if certain electrically neutral objects are rubbed, they can become charged. For example; when rubber is rubbed with a wool cloth, both become charged. or a comb through hair The rubber scrapes electrons from fur atoms. So the rubber is negatively charged and the cloth is positively charged.
Ben also knew that a charge separation occurs when a glass rod is rubbed with a silk cloth In the case of the glass and silk, the glass rod loses negative charge and becomes positively charged while the silk cloth gains negative charge and therefore becomes negatively charged.
Ben experimented with the interactions between the charge objects. He suspended one and brought other charged objects near repel attract repel Electrons: Ben observed that like charged object repel and unlike charges attract
Fundamental Rule Opposites attract, Likes Repel Things don t like having a net charge If objects don t like having a net charge, then how does it happen?
Silk on glass glass (+) Who loses electrons? The glass Fur on rubber rubber (-) Who loses electrons? The fur
When objects get Charged: Must obey Law of Conservation of Charge Charges may be transferred among different atoms, materials, or objects but all charge is accounted for. NO NEW charges are created nor are any charges destroyed. Only electrons can move
Remember: An excess of electrons results in: A negative charge A shortage of electrons results in: A positive charge ONLY ELECTRONS MOVE
How Do Charges Behave in Materials? Conductor: Allows electrons to move easily Metals, why? Metals lose electrons, (not held tightly) Insulator: Does not allow electrons to move easily Non metals. Glass, plastic, dry wood. Why? Electrons held tightly
Semiconductors charges only move freely when certain conditions are met (i.e., heat, sufficient voltage, etc.) ex germanium, selenium, and silicon. Superconductors charges move effortlessly and cannot be stopped once they are moving
Objects become charged by Friction Electrons are rubbed off one insulator onto another insulator Conduction Induction Grounding With a credit card
FRICTION: e- rubbed off one insulator to another 16
Objects become charged by Friction Conduction Charging by CONTACT with a charged object Induction Grounding
Charging by Conduction Requires Contact Electrons transferred. Results in: Object with the same charge as original charged object. Some electrons leave rod and spread over sphere.
Objects become charged by Friction Conduction Induction Charging an object WITHOUT touching a charged object Grounding
Induction no contact occurs between charged object and neutral object.. Involves temporary rearrangement of electrons on neutral object Neutral Object becomes polarized but net charge remains the same If neutral polarized object is grounded, charge will become opposite of the charged object and is no longer temporary 20
Charging by Induction Neutral objects can be temporarily attracted to charged objects by a process called POLARIZATON.
A negatively charged balloon is brought near a neutral conducting sphere as shown below. As it approaches, charge within the sphere will distribute itself in a very specific manner. Which one of the diagrams below properly depicts the distribution of charge in the sphere?
What is grounding? Involves Transfer of excess electrons to and from the ground
Charging by Induction AND Grounding polarization grounding permanent charge The rod does not touch the sphere. It pushes electrons out of the back side of the sphere and down the wire to ground. The ground wire is disconnected to prevent the return of the electrons from ground, then the rod is removed. The charge on the object is opposite if grounded
How can I tell if something is charged?
Pith Ball Experiment Pith Ball and a Charged Rod Pith Ball Experiment 26
In the beginning, the negative rod repels the electrons in the neutral pith ball to the ight. This causes the left side of the pith ball to obtain a net positive charge. Since opposite charges attract, the pith ball is attracted to the rod. When they touch, electrons are transferred to the pith ball giving it a net negative charge. Since the rod remains negative even after the transfer, it repels the negatively 27
Electroscope -detects the presence of charge When the leaves are charged, what will happen? When the leaves are not charged, what will happen?
An electroscope is a device that detects static charge. Negatively charged Positively charged The metal leaves of the electroscope move apart if a charged object is brought near the knob. Benjamin Franklin used a similar device when he investigated charges.
To review Induction results in an OPPOSITE CHARGE Conduction results in the SAME CHARGE
Grounding is allowing charges to move freely along a connection between a conductor and the ground. The Earth (the ground) is a practically infinite reservoir of electric charge. Here a positively charge rod attracts electrons from the ground into the electroscope Here a negatively charge rod repels electrons into the ground from the sphere
Static Electricity in our lives Why are there more problems with static during the dry winter months? The net charge of a water molecule is neutral, however it is a polar molecule. It can attract a build up of excess electrons Some Static Electricity is due to friction Clothes stick to each other in the dryer How to solve this problem? Dryer sheets. How do they work? Dryer sheets contain lots of polar molecules to absorb the excess electrons and keep clothes neutral
Lightning Becomes very negative. Becomes very positive. WHO DETERMINED POSITIVE AND NEGATIVE? 33
How lightning forms YouTube how lightning works YouTube The Birth of a Lightning Bolt - YouTube
Four fundamental forces in nature Gravity Weak nuclear Electromagnetic (electricity and magnetism) Strong nuclear
Electrical Forces Why don t we notice them? The attractive and repulsive forces between the charges in Earth and the charges in your body balance out!!!!
Electromagnetic force is significant at atomic level. Things you know: Atom has positively charged nucleus surrounded by negatively charged electrons All electrons are identical (same mass; same quantity of negative charge) All protons are identical (same mass; same quantity of positive charge) Nucleus composed of protons and neutrons. Neutrons are neutral Neutral atoms have equal protons and electrons so zero net charge
What exactly is CHARGE? It is physical property of matter. It comes in two flavors: plus and minus. What is the unit for charge? Coulombs (C) 39
What is the smallest charge possible? Millikan Oil Drop Experiment In 1910, Millikan was able to measure the charge of an electron. The smallest charge possible is: -1.602 x 10-19 Coulombs (C). 40
Definition of Coulomb Abbreviation: C SI unit for charge One coulomb is NOT equal to the charge of 1 electron!!!! 1C ~ the charge of 6.25 x 10 18 electrons It is the amount of charge to pass through a cross-section of wire in 1 second when 1 Ampere (A) of current is applied. (We ll cover the amp later.) Likewise the + charge of protons is associated with 6.25 x 10 18 protons 41
Elementary Particles Particle Charge, (Coulombs per particle) # of particles in a Coulomb electron -1.6 x 10-19 6.25 x 10 18 proton +1.6 x 10-19 6.25 x 10 18 42
Ex 1 A strong lightning bolt transfers 35 C to Earth. How many electrons were transferred? (35 C) (6.25 x 10 18 electrons) 2.19 x 10 20 electrons
Coulomb s Law Charles-Augustin de Coulomb used a torsion pendulum to establish his law. F k q q 1 d 2 2 rˆ 44
Electric Force F k q q d q charge, C (coulombs) 1 rˆ d distance between charges, m F electric force, N k electrostatic constant 9.00 x 10 9 Nm 2 /C 2 2 2 45
What happens to F as charge increases? Increase What happens to F as r increases? Decreases by inverse square Look at k c. Is this a large or small value? large How is q described for a proton? positive For an electron? negative
The Product of q 1 and q 2 If the product, q 1 q 2,is negative then the force is attractive. If the product, q 1 q 2,is positive then the force is repulsive. 47
Ex 2: Two negatively charged balloons are 0.70m apart. If the charge of each is 2.0 x 10-6 C, What is the electric force between the two balloons? q 1 = q 2 = 2.0 x 10-6 C d = r = 0.70 m F k q 1 q 2 r 2 F = 9.0 x 10 9 N m 2 /C 2 (-2.0 x 10-6 C) 2 (0.70m) 2 F = 0.073 N An attracting or repelling force?
Ex.3: Two equally charged balloons repel each other with a force of 4.0 x 10-3 N. If they are 0.015 m apart, what is the charge of the each balloon? F = 4.0 x 10-3 N F k q q d = 0.015 m q 2 = Fd 2 k q 2 = (4x10-3 N)(0.015m) 2 (9x10 9 Nm 2 /C 2 ) q 1 = q 2 = 1.0 x 10-8 C 1 2 2 r
Ex 4: How many Coulombs are in a µc? 1 x 10-6 Two charges are separated by 3.0 cm. Object A has a charge of +6.0 µc. Object B has a charge of -6.0 µc. What is the force on Object A? Is the force attractive or repelling? -360N, attractive
Ex 5 Two electrons exert an electrical force of 1.0 x 10-8 N on one another. Is this an attractive or repelling force? Repelling Calculate the distance between them. Rearrange formula to solve for d Use known charge for an electron 1.5 x 10-10 m
Two charges create a force on one another. If the charge of one object is doubled, how does the resulting force change? F will double What if charge of one object is tripled? F will triple F k q q 1 d 2 2 rˆ
Two charges create a force on one another. If the distance between the objects is increased by a factor of 2, the force changes by a factor of? F will decrease by a factor of 4 What if distance between the objects is tripled? F will decrease by a factor of 9 F k q q 1 d 2 2 rˆ
Review. How many electrons in one Coulomb? 6.25 x 10 18 electrons What is the charge of one electron -1.6 x 10-19 Coulombs (C) How many protons in one Coulomb? 6.25 x 10 18 protons What is the charge of one proton +1.6 x 10-19 Coulombs (C).
Review. How many electrons in one Coulomb? 6.25 x 10 18 electrons Calculate the charge of one electron -1.6 x 10-19 Coulombs (C) How many protons in one Coulomb? 6.25 x 10 18 protons Calculate the charge of one proton +1.6 x 10-19 Coulombs (C).
Force and Fields Contact forces What we mostly deal with Objects touch each other directly Ex. A tennis racket hits a tennis ball F=ma www.cartoonstock.com.
Forces can occur without contact! Action at a distance Can you think of anything that applies a force without touching? 57
Gravity demonstrates action at a distance What happens if you get too far away from the mass exerting the force? The effects are less 58
What else applies an action at a distance? Magnets! 59
What else applies an action at a distance? 60
Attracting and repelling forces of charges 61
The space that surrounds these things is altered Examples: Magnets Sun Planets Electric charge
Action at a distance depends on a field of influence An object within the field may be affected by it Can be scalar or vector Magnitude only Ex. Heat Can be vector Magnitude and direction Ex. Gravity (one direction only since only attracts) Ex. Electric (more than one direction; attracts and repels 63
Fields are NOT Force, they exert the force Ex. A person pushes a box. The person is not the force, he exerts the force! 64
Electric field A field that exerts force that surrounds an electric charge or group of charges Magnitude and direction (vector)
Electric field How would you detect and measure an electric field around a charge? Place another one nearby and see what happens! Since all charges produce fields, come up with a model 66
Electric field model Source charge: charge producing the field. Usually designated with a capital Q Test charge: a mathematical creation Always positive Symbol: q Doesn t exist Infinitely small, thus produces no field of its own 67
What is the source charge if The test charge q moves towards it? Negative (attracts) The test charge q moves away from it? Positive (repels) How would I draw these? 68
Where do you think the field is strongest? 69
What if I had more than one source charge? What would the field lines look like? 70
Think: Where is the electrical potential energy of a positive test charge (q+) higher, at the point A or B? Why? Point A. Because of it s location, it is not where it wants to be. It took work to get it there!
The electric field is strongest in regions where the lines are close together and weak when the lines are further apart.
These fields can be detected in lab Threads floating on oil bath become polarized and align themselves with the electric field.
How do I measure the strength of the electrical field around a source charge (Q)? What factors do you think the electrical field strength is dependent on? 1. Force (Push or Pull) of Source Charge on Test Charge 2. Distance Between Source Charge and Test Charge
First let s consider effect of force 76
Electric Field Intensity (Strength) E F q' E - Electric Field Strength or Intensity (N/C) F - Force experienced by a test charge at that location (N) q - magnitude of the test charge placed at that location (C).
Ex 6 A test charge has a magnitude of 1 x 10-10 C. It experiences a force of 2N in an electrical field. What is the Intensity of the field? Variables: F = 2N q = 1 x 10-10 C E = F/q E = 2N/ 1 x 10-10 C E = 2 x 10 10 N/C
Ex 7 A test charge of 6 x 10-26 uc is placed 200 mm from a proton (this is the source charge). What is the electrical force between them? What is the Field strength at this point? What is the direction of the field? Variables: q = 6 x 10-26 µc p+= 1.6 x 10-19 C d = 200 mm Variables: q = 6 x 10-32 C p+= 1.6 x 10-19 C d = 0.2m Now we can solve. This is a 2 step problem. Step 1: Solve for force using Coulombs Law Step 2: Use the calculated force and solve for Field Intensity
Ex 7 cont A test charge of 6 x 10-26 µc is placed 200 mm from a proton (this is the source charge). What is the electrical force between them? What is the Field strength at this point? What is the direction of the field? Variables: q = 6 x 10-32 C p+= 1.6 x 10-19 C d = 0.2m F = kqq d 2 F = (9 x 10 9 C)(6 x 10-32 C)(1.6 x 10-19 C) 0.2m 2 F = 2.16 x 10-39 N E = F/q E = 2.16 x 10-39 N / 6 x 10-32 C E = 3.6 x 10-8 N/C
Remember: The direction of the electric field at a point in space is the same as the direction in which a positive charge would move if it were placed at that point. The electric field lines or lines of force indicate the direction. + -Q Electric field line flow Out of positive charges and into Negative charges.
The electric field intensity E at a distance d from a source charge Q can be found without knowing the test charge!: Units: N/C
EX 8: What is the electric field intensity at a distance of 2 m from a source charge of -12 μc? Include direction. d = 2 m q = -12 μc q = -12μC To determine the direction of the field, ask If the source charge is negative do the field lines go out or in? E kq d 2 9 6 9x10 ( 12x10 ) = 2.7x10 4 N/C, towards q 2 2 or to the left
How do I determine the field strength if there are multiple charges? 84
When more than one charge contributes to the field, the resultant field is the vector sum of the contributions from each charge. E kq d 2 Where k : 9x10 9 Nm 2 /C 2 Units: N/C Note we will look at direction of the field to know whether fields add or subtract at a point.
Remember this? + -Q Electric field line flow Out of positive charges and into Negative charges.
Ex 9: Two charges q 1 =-8 μc and q 2 =+12 μc are placed 120 mm apart in the air. What is the electric field at the midpoint between them? q 1 = -8 μc q 2 = +12 μc r = 0. 120m - E T E 1 E 2 X q 1 + q 2 E kq d 2 = kq 1 + kq 2 r 2 r 2 = (9 x 10 9 )(8 x 10-6 ) + (9 x 10 9 )(12 x 10-6 ) (0.06) 2 (0.06) 2 E= 2.0 x 10 7 + 3 x 10 7 = 5.0 x 10 7 N/C to the left
Ex.10: Two charges q 1 =+8 μc and q 2 =+12 μc are placed 120 mm apart in the air. What is the electric field at the midpoint between them? q 1 = + 8 μc q 2 = +12 μc r = 0. 120m + E T E 1 E 2 X q 1 + The fields are in opposite directions so they subtract q 2 E kq = kq 1 - kq 2 r 2 r 2 r 2 = (9 x 10 9 )(8 x 10-6 ) - (9 x 10 9 )(12 x 10-6 ) (0.06) 2 (0.06) 2 E= 2.0 x 10 7-3 x 10 7 = -1.0 x 10 7 N/C E = 1.0 x 10 7 N/C to the left